In the operation of direct current motors for servo actuators, it is desirable and even necessary in certain applications to eliminate or prevent continued rotation of the motor rotor after power to the motor has been switched off. This function has been found particularly necessary in low voltage servo motor applications as, for example, those found in certain automotive accessory applications such as power-operated windows, power seats and pedal adjustment mechanisms.
In order to effect dynamic braking of a direct current motor rotor, it has been found necessary to find a way or means of using the counter EMF of the rotating motor magnets after the current supply to the stator coil is cut to provide braking of the continued rotation of the motor rotor.
Heretofore, dynamic braking of a servo motor has been accomplished by the use of two independent single pole, double throw switches with the normally closed contacts of both switches connected to the negative or ground side of a direct current servo motor circuit, with the normally open contacts connected to the B+ supply voltage by having each common pole connected to a motor terminal. When the switch is actuated for servo motor rotation in one direction, one of the switches changes state while the other switch mechanism remains unchanged. For reverse servo motor rotation, the other of the two switch mechanisms is actuated while the one or first switch mechanism remains unchanged. Thus, the switching arrangements of the prior art for dynamic braking of a user control reversibly operated direct current servo motor required independent actuation of one or the other of two separate independent switching mechanisms.
Referring to FIGS. 6 and 7, the prior art switching arrangement for dynamic braking of direct current servo motors is shown wherein a single pole, double throw switch assembly indicated generally at 1 has a normally closed contact 2, a normally open contact 3 connected to the B+ supply and a common contact blade 4 connected to one motor coil terminal. A normally closed contact 2 is grounded and the switch is actuated by user operated plunger 5. It will be understood that two of the switch arrangements denoted by reference numeral 1 are required for forward and reverse operation of the servo motor.
Referring to FIG. 7 the switch 1 is shown connected to one motor coil 6 through the common terminal; and, a second identical switch 1' is connected through its common terminal to a second motor coil disposed to effect opposite direction motor rotation from coil 6. The stationary normally closed contact 2 for switch 1 is grounded as is the normally closed contact 2' of switch 1'. Upon movement of the plunger 5 in one direction switch 1 is actuated to energize coil 6; and, whereas movement of the second actuator 5' must be effected to cause energization of coil 7 for reverse motor operation. Thus the switching arrangement of the prior art requires two individual single pole, double throw switches which requires additional components and results in relatively higher cost installations, particularly for motor vehicle window lift motor controls.